[R] Sampling from a Population

[Lorenzo Isella]

Dear All,
I hope this is not too off topic, but I am sure it has to be a one-liner  
in R.
Suppose you have a population of size N and that you take a random sample  
of n_s individuals out of this population.
This population includes a subgroup of n_i individuals.
For any individual in n_i, what is the probability of being included in  
the sample n_s?
Many thanks.

Lorenzo